High-voltage power unit and mass spectrometer using the power unit

ABSTRACT

The output terminal P 2  of the positive voltage generating circuit  2  and the output terminal Q 1  of the negative voltage generating circuit  4  are connected in series. The output terminal Q 2  of the negative voltage generating circuit  4  is connected to the ground via the resistor  10.  Each of the resistors  6  and  7  is respectively connected in parallel to the voltage generating circuits  2  and  4.  A high voltage whose polarity is changed is taken from the output terminal P 1  of the positive voltage generating circuit  2.  For changing the output voltage from positive to negative one, the control circuit  1  controls each of the drive circuits  3  and  5  so that the output of the positive voltage generating circuit  2  changes from the voltage +HV to zero and the output of the negative voltage generating circuit  4  simultaneously changes from zero to −HV. Accordingly, the output voltage changes in a short period of time. Although a high-voltage-resistance resistor is required in this configuration, it is far more inexpensive and has higher reliability compared to a high-voltage relay or a semiconductor switch which has been conventionally used.

TECHNICAL FIELD

The present invention relates to a high-voltage power unit capable ofswitching the polarity of an output voltage between positive andnegative, and to a mass spectrometer using such a power unit.

BACKGROUND ART

Conventionally, ion detectors including a conversion dynode and asecondary-electron multiplier have been used for detecting ions withhigh sensitivity in a mass spectrometer. In such an ion detector, a highvoltage (up to approximately ±10 kV for example) having a polarityopposite to that of the ions to be analyzed is required to be applied toa conversion dynode for selectively detecting positive ions and negativeions. In a liquid chromatograph mass spectrometer, an ion sourceprovided by an electrospray ionization (ESI) method is used for examplefor vaporizing and simultaneously ionizing a liquid sample. In such anion source, a high voltage (approximately ±several kV for example) witha polarity equal to that of the ions to be analyzed is required to beapplied to the tip of a nozzle for spraying a liquid sample.

In such a use as described earlier, the polarity of the high voltageapplied is required to be changed corresponding to whether the object tobe analyzed is a positive ion or a negative ion. Therefore, ahigh-voltage power unit capable of switching the polarity of an outputvoltage of single system is used. One of the most conventionallywell-known high-voltage power units for switching high voltages ofdifferent polarity is one using a high-voltage reed relay (See PatentDocument 1 for example.).

In a high-voltage power unit with such a configuration, when changing arelay, spike discharges may occur to break the relay. Therefore, whenswitching the polarity of an output voltage, it is necessary to observethe following procedure: decrease the output voltage, actuate the relayto change the contacts when the output voltage becomes adequately small,and after that, increase the output voltage. Consequently, it takes sometime to change the polarity, which elongates the non-detection period.This affects the accuracy of an analysis in the case where, for example,the detection of the positive ions and negative ions are alternatelyperformed by switching them within a short period of time in a massspectrometer. In addition, a high-voltage relay of 10 kV or above isgenerally expensive but nonetheless does not have a very long life time.That is, it is costly and also has a problem in reliability.

On the other hand, a high-voltage semiconductor switch (e.g. powerMOSFET or the like) might be used in place of a reed switch. Theswitching operation of a semiconductor switch itself is much faster thanthat of a relay. However, although a high-voltage semiconductor switchof 10 kV or above is available, it is highly expensive. Even in caseswhere relatively-inexpensive semiconductor switches are used, since theyhave to be connected in series in multiple stages, the cost eventuallybecomes significantly high. And a semiconductor switch also has aproblem in reliability since it is weak in the discharge of a highvoltage.

[Patent Document 1] U.S. Pat. No. 6,002,600 (refer to FIG. 1C)

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The present invention is achieved to solve the previously-describedproblem, and the objective thereof is to provide a high-voltage powerunit capable of reducing the cost more than before with high reliabilityand also capable of relatively-quickly changing the polarity of anoutput voltage, and to provide a mass spectrometer using such a powerunit.

Means for Solving the Problems

To solve the previously-described problem, the present inventionprovides a high-voltage power unit capable of selectively providing ahigh voltage of positive or negative polarity, including:

a positive voltage generating circuit for generating a positive highvoltage;

a negative voltage generating circuit for generating a negative highvoltage;

a control circuit for controlling each of the positive high voltagegenerated by the positive voltage generating circuit and the negativehigh voltage generated by the negative voltage generating circuit;

a first resistor connected in parallel to an output side of the positivevoltage generating circuit; and

a second resistor connected in parallel to an output side of thenegative voltage generating circuit,

wherein:

one output terminal of the positive voltage generating circuit and oneoutput terminal of the negative voltage generating circuit are connectedto connect the two voltage generating circuits in series connection; and

one of two terminals of the series connection is used as a referenceside and a high-voltage output whose polarity is changed is taken outfrom the other terminal.

As one exemplary embodiment of the high-voltage power unit according tothe present invention, each of the positive voltage generating circuitand the negative voltage generating circuit includes a boostertransformer, and a rectifier circuit using a Cockcroft-Walton circuitconnected to a secondary winding of the transformer.

Effect of the Invention

In a high-voltage power unit according to the preset invention, in orderto switch the positive and negative high voltages, there is no need forusing high-voltage reed relays or semiconductor switches as in aconventional manner; only a high-voltage-resistance resistor is addedand connected in parallel on the output side of each voltage generatingcircuits. Therefore, a significant cost reduction can be achieved. Inaddition, a discharge accompanying the switching does not occur, and thebreakage of an element thereby does not take place. Even in the casewhere a spike noise or the like are included in the high-voltage outputfor some reason, a breakage does not easily occur, unlike asemiconductor switch or the like. Accordingly, a high reliability can beassured for a long period of time.

In the high-voltage power unit according to the present invention, thecontroller controls the positive voltage generating circuit and thenegative voltage generating circuit, when switching the polarity of thehigh-voltage output between positive and negative, so that when anoutput voltage of either one of the voltage generating circuits changesfrom a first predetermined voltage to zero, an output voltage of theother voltage generating circuit simultaneously changes from zero to asecond predetermined voltage.

That is, there is no need to decrease the output voltage of one side andthen increase the output voltage of the other side in changing thepolarity. The control as described earlier can bring about a rapidpolarity change.

The high-voltage power unit according to the present invention mayfurther include a voltage-dividing resistor for dividing thehigh-voltage output, wherein a detected voltage by the voltage-dividingresistor is sent back to the control circuit as a feedback, and thecontrol circuit refers to the detected voltage to control the positivevoltage generating circuit and/or the negative voltage generatingcircuit.

With this configuration, it is possible to make the voltage value of ahigh-voltage output converge to a target voltage with a high degree ofaccuracy by a closed feedback loop. Hence, the voltage value of ahigh-voltage output will stay stable even in the case where some sort ofdisturbance, i.e. load fluctuation, occurs.

The high-voltage power unit as described earlier can be used for variouspurposes and applied to various apparatuses; for example, a massspectrometer. Specifically, for example, the high-voltage output by thehigh-voltage power unit may be used in an ion source and/or an iondetector. In this case, the polarity of the high-voltage output by thehigh-voltage power unit may be switched corresponding to the polarity ofan ion to be analyzed.

Since the high-voltage power unit according to the present invention canquickly change the polarity as stated earlier, it is possible to shortenthe non-detection period in which both positive and negative ions cannotbe detected, and the detection failure can be therefore decreased in thecase where, for example, it is necessary to alternately perform thepositive ion's detection and the negative ion's detection every shortperiod of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of the main portion of a high-voltagepower unit according to an embodiment of the present invention.

FIG. 2 is a detail configuration diagram of the drive circuit in FIG. 1.

FIG. 3 is a waveform diagram illustrating the states when the polarityis changed in the high-voltage power unit of the present embodiment.

FIG. 4 is a schematic diagram illustrating the current flow in thehigh-voltage power unit of the present embodiment.

FIG. 5 is a schematic configuration diagram of a mass spectrometer usingthe high-voltage power unit of the present embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one embodiment of a high-voltage power unit according tothe present embodiment will be described in detail in reference to FIGS.1 through 4. FIG. 1 is a configuration diagram of the main portion ofthe high-voltage power unit according to the present embodiment.

This high-voltage power unit provides a positive or negative DC highvoltage of approximately a few to 10 kV and includes a positive voltagegenerating circuit 2, a negative voltage generating circuit 4, and acontrol circuit 1 for controlling the drive circuits 3 and 5 included inthe voltage generating circuits 2 and 4. The positive voltage generatingcircuit 2 and the negative voltage generating circuit 4 basically havethe same configuration. For example, the positive voltage generatingcircuit 2 includes a transformer T1 as a booster, a drive circuit 3 fordriving the primary winding of the transformer T1, and a rectifiercircuit (voltage quadrupler rectifier circuit) using a Cockcroft-Waltoncircuit composed of four capacitors C1 through C4 and four diodes D1through D4 connected to the secondary winding of the transformer T1. Inthe negative voltage generator 4, the direction of each of the diodes D5through D8 in the Cockcroft-Walton circuit is opposite to that in thepositive voltage generator 2.

The output terminal P2 of the positive voltage generating circuit 2 andthe output terminal Q1 of the negative voltage generating circuit 4 areconnected. The other output terminal Q2 of the negative voltagegenerating circuit 4 is connected to the ground (reference zeropotential) via a resistor 10. This resistor 10 is placed only for thecurrent monitor, and the operation of this circuit is basically the sameas in the case where the resistor 10 is shunted. Hence, the outputterminal Q2 corresponds to the reference side in the present invention.Between the output terminals P1 and P2 of the positive voltagegenerating circuit 2, a resistor 6 which corresponds to the firstresistor of the present invention is connected in parallel, and betweenthe output terminals Q1 and Q2 of the negative voltage generatingcircuit 4, another resistor 7 which corresponds to the second resistorof the present invention is connected in parallel. That is, theresistors 6 and 7 also have the form of a series connection. And, ahigh-voltage output is taken out from the resistor 6's terminal that ison the other side of the node between the resistors 6 and 7, i.e. fromthe output terminal P1 of the positive voltage generating circuit 2. Aresistor 8 and a resistor 9 are connected in series between thehigh-voltage output terminal and the ground. And from a node between theresistors 8 and 9, a monitor output is taken out, which is provided tothe control circuit 1 as a feedback.

FIG. 2 illustrates the interior configuration of the drive circuits 3and 5. In the drive circuits 3 and 5, a DC voltage is applied to aprimary winding of the transformer T1 by the DC voltage source 21. Thisapplication voltage (or a supply current) is turned on/off by aswitching element 22 connected in series to the primary winding. Theswitching element 22 is switched on/off by a control signal supplied bythe switching (SW) element driver 23. When the pulse width of arectangular wave signal generated in the SW element driver 23 changes,the effectual electric power supplied to the primary winding of thetransformer T1 changes. Accordingly, it is possible to change the outputvoltage of the positive voltage generating circuit 2 and the negativevoltage generating circuit 4. Alternatively, the application voltageitself of the DC voltage source 21 may be changed with a control signal;the effective electric power supplied to the primary winding of thetransformer T1 is accordingly changed, and consequently the outputvoltage of the positive voltage generating circuit 2 and the negativevoltage generating circuit 4 is changed.

An operation of the high-voltage power unit according to the presentembodiment when switching the polarity of the voltage provided from thehigh-voltage output terminal will be described in reference to FIGS. 3and 4. FIG. 3 is a waveform diagram illustrating the states when thepolarity is switched, and FIG. 4 is a schematic diagram illustrating thecurrent flow in the circuit.

When a positive voltage +HV is provided to the high-voltage outputterminal, a voltage is intermittently applied to the primary winding ofthe transformer T1 in the positive voltage generating circuit 2 underthe control of the control circuit 1, which makes the output voltage ofthe positive voltage generating circuit 2 +HV. On the other hand, novoltage is applied to the primary winding of the transformer T2 of thenegative voltage generating circuit 4, hence the output voltage of thenegative voltage generating circuit 4 is 0. In this case, as illustratedin FIG. 4( a), a current i flows from the ground through the resistor10, the resistor 7, and the positive voltage generating circuit 2, andis consequently supplied to a load connected to the high-voltage outputterminal. Since the resistor 7 is connected in parallel to the negativevoltage generating circuit 4, the current pathway as described earliercan be formed. Since a voltage value corresponding to the voltage +HVappearing in the high-voltage output terminal is provided to the controlcircuit 1 as a feedback, the control circuit 1 compares this voltagevalue with a target value, and changes the control signal for the SWelement driver 23 in the positive voltage generating circuit 2 orchanges the application voltage of the DC voltage source 21 in order toreduce the error. Accordingly, it is possible to precisely set theoutput voltage +HV to any target voltage.

When the output of the high-voltage output terminal is changed from thepositive voltage +HV to the negative voltage −HV, the control circuit 1stops applying a voltage to the primary winding of the transformer T1 inthe positive voltage generating circuit 2, and at the same time, sends acontrol signal to the drive circuits 3 and 5 so that a voltage isstarted to be intermittently applied to the primary winding of thetransformer T2 in the negative voltage generating circuit 4. Thiscontrol change can be simultaneously performed, for example, at theprecision of the time ta in FIG. 3. Based on such a control change, anapplication voltage to the primary winding of transformers T1 and T2immediately changes; however, since a capacitor in the rectifier circuitis required to be charged or discharged, the output of each of thevoltage generating circuits 2 and 4 gradually changes. As illustrated inFIGS. 3( a) and (b), the positive voltage gradually decreases from +HVto zero, and in contrast, the negative voltage gradually increases fromzero to converge on −HV after a slight overshoot (to negative voltages).

The voltage appearing on the high-voltage output terminal has a waveformin which the positive voltage and the negative voltage as describedearlier are added. Therefore, it changes from the positive voltage +HVto the negative voltage −HV in a short changing time t1 as illustratedin FIG. 3( c). With a conventional configuration using a reed relay, thechanging time is fairly long since it is necessary to increase thenegative voltage after the positive voltage has decreased to zero.Contrary to this, the configuration of the present embodiment candramatically shorten the changing time. Specifically, for example,although it takes approximately 400 ms for the changing time in aconventional configuration, it takes 10-20 ms for the changing time withthe configuration of the present embodiment.

When a negative voltage is provided to the high-voltage output terminal,as illustrated in FIG. 4( b), a current flowing through the loadconnected to the high-voltage output terminal flows into the ground viathe resistor 6, the negative voltage generating circuit 4, and theresistor 10. Since the resistor 6 is connected in parallel to thepositive voltage generating circuit 2, the current pathway as describedearlier can be formed. Since a voltage value corresponding to thevoltage −HV appearing in the high-voltage output terminal is provided tothe control circuit 1 as a feedback even in this case as a matter ofcourse, the control circuit 1 compares this voltage value with a targetvalue, and changes the control signal for the SW element driver 23 inthe negative voltage generating circuit 4 or changes the applicationvoltage of the DC voltage source 21 in order to reduce the error.Accordingly, it is possible to precisely set the output voltage −HV toany target voltage.

The operation for changing the output of the high-voltage outputterminal from the negative voltage −HV to the positive voltage +HV isbasically the same, as described earlier, as in the case where theoutput is changed from the positive voltage +HV to the negative voltage−HV. Therefore, the changing time is very short.

With the configuration of the high-voltage power unit according to thepresent embodiment, a discharge or the like does not take place whenchanging the voltage as previously described. Therefore, a breakage orfailure may not occur unlike in the case where a semiconductor switch isused, and a high reliability can be maintained. In addition, since ahigh-voltage-resistance resistor is far more inexpensive compared to ahigh-voltage reed switch or a semiconductor switch, it also has a greatadvantage in cost.

In the previously-described configuration, the resistance value of theresistors 6 and 7 which are connected in parallel to the voltagegenerating circuits 2 and 4 is important. Because, if the resistancevalue is too high, the voltage drop in the resistors 6 and 7 in thestates illustrated in FIGS. 4( a) and (b) becomes large, and the powerloss in a steady state accordingly becomes large. On the other hand, ifthis resistance value is too small, the proportion of the currentflowing into the resistors 6 or 7 in the current that should primarilyflow into the load rises, which also leads to a power loss. Therefore,it is desirable to properly set the resistance value of the resistors 6and 7 by the voltage value of a high-voltage output, the load'scondition, etc. When a high voltage of 10 kV is provided, for instance,each resistance value of the resistors 6 and 7 may be set toapproximately 50 MΩ. The resistance value of the resistor 8 as a divideris required to be large: it may be set to 1 GΩ for example.

FIG. 5 illustrates an example of a schematic configuration of a massspectrometer using the high-voltage power unit according to the presentembodiment. This mass spectrometer is used for analyzing a sample liquideluted from a liquid chromatograph for example. The sample liquid issprayed into an atmosphere of approximate atmospheric pressure from thenozzle 31 for an electrospray. This ionizes sample atoms and moleculesincluded in the sample liquid. In this process, high voltages (normallyseveral kV) with different polarities according to the polarity of theions to be generated are required to be applied to the tip of the nozzle31. As a voltage source therefor, the high-voltage power unit 37 of apolarity changing type as described earlier can be used.

The ions generated by the aforementioned ionization are sent to asubsequent stage through a desolvation pipe 32, and are converged by anion lens 33 driven by a voltage source, which is not illustrated, to beintroduced into a space across the long axis of a quadrupole filter 34.A voltage in which an RF voltage and a DC voltage are superposed isapplied to the quadrupole filter 34 by the voltage source which is notillustrated. And by the electric field formed by the voltage, only ionshaving a predetermined mass number pass through the quadrupole filter34. A high voltage (normally, approximately 10 kV) with a polarityopposite to that of the ions to be analyzed is applied to a conversiondynode 35 in the further subsequent stage by a high-voltage power unit38 of a polarity changing type as previously described. The ions inducedby the electric field by the high voltage touch the conversion dynode 35to beat out secondary electrons. The secondary electrons emitted flydownward and reach a secondary electron multiplier 36. They aremultiplied inside the secondary electron multiplier 36, and a detectionsignal corresponding to the number of the secondary electrons that flewin first, i.e. the number of the ions that reach the conversion dynode35, is taken out.

That is, in the mass spectrometer with the previously-describedconfiguration, the controller 39 sends control signals for switching toeach of the high-voltage power units 37 and 38 in accordance with thepolarity of the ions to be analyzed. Since the polarity changing in thehigh-voltage power units 37 and 38 can be quickly performed as describedearlier, even in the case where, for example, the positive ions'detection and negative ions' detection are repeatedly performedalternately every short period of time, the time in which ion detectioncannot be performed is shortened; accordingly, fine mass chromatogramsand total ion chromatograms can be created.

It is apparent that FIG. 5 is an example of a simplified configurationand the previously-described high-voltage power unit of a polaritychanging type may be used in a part other than that described earlier.

The configurations of the high-voltage power unit described in thepreviously-described embodiments is merely an example of the presentinvention, and it is clear that any modifications, additions andadjustment to be made appropriately within the scope of the presentinvention are also included in the scope of the claims of the presentapplication.

1. A high-voltage power unit capable of selectively providing a highvoltage of positive or negative polarity, comprising: a positive voltagegenerating circuit for generating a positive high voltage; a negativevoltage generating circuit for generating a negative high voltage; acontrol circuit for controlling each of the positive high voltagegenerated by the positive voltage generating circuit and the negativehigh voltage generated by the negative voltage generating circuit; afirst resistor connected in parallel to an output side of the positivevoltage generating circuit; and a second resistor connected in parallelto an output side of the negative voltage generating circuit, wherein:one output terminal of the positive voltage generating circuit and oneoutput terminal of the negative voltage generating circuit are connectedto connect the two voltage generating circuits in a series connection;and one of two terminals of the series connection is used as a referenceside, and a high-voltage output whose polarity is changed is taken outfrom the other terminal.
 2. The high-voltage power unit according toclaim 1, wherein each of the positive voltage generating circuit and thenegative voltage generating circuit includes a booster transformer, anda rectifier circuit using a Cockcroft-Walton circuit connected to asecondary winding of the transformer.
 3. The high-voltage power unitaccording to claim 1, wherein the controller controls the positivevoltage generating circuit and the negative voltage generating circuit,when switching a polarity of the high-voltage output between positiveand negative, so that when an output voltage of either one of thevoltage generating circuits changes from a first predetermined voltageto zero, an output voltage of the other voltage generating circuitsimultaneously changes from zero to a second predetermined voltage. 4.The high-voltage power unit according to claim 1, further comprising avoltage-dividing resistor for dividing the high-voltage output, whereina detected voltage by the voltage-dividing resistor is sent back to thecontrol circuit as a feedback, and the control circuit refers to thedetected voltage to control the positive voltage generating circuitand/or the negative voltage generating circuit.
 5. A mass spectrometerusing the high-voltage output by the high-voltage power unit accordingto claim 1 in an ion source and/or an ion detector, wherein a polarityof a high-voltage output by the high-voltage power unit is switchedcorresponding to a polarity of an ion to be analyzed.
 6. Thehigh-voltage power unit according to claim 2, wherein the controllercontrols the positive voltage generating circuit and the negativevoltage generating circuit, when switching a polarity of thehigh-voltage output between positive and negative, so that when anoutput voltage of either one of the voltage generating circuits changesfrom a first predetermined voltage to zero, an output voltage of theother voltage generating circuit simultaneously changes from zero to asecond predetermined voltage.
 7. The high-voltage power unit accordingto claim 2, further comprising a voltage-dividing resistor for dividingthe high-voltage output, wherein a detected voltage by thevoltage-dividing resistor is sent back to the control circuit as afeedback, and the control circuit refers to the detected voltage tocontrol the positive voltage generating circuit and/or the negativevoltage generating circuit.
 8. The high-voltage power unit according toclaim 3, further comprising a voltage-dividing resistor for dividing thehigh-voltage output, wherein a detected voltage by the voltage-dividingresistor is sent back to the control circuit as a feedback, and thecontrol circuit refers to the detected voltage to control the positivevoltage generating circuit and/or the negative voltage generatingcircuit.
 9. A mass spectrometer using the high-voltage output by thehigh-voltage power unit according to claim 2 in an ion source and/or anion detector, wherein a polarity of a high-voltage output by thehigh-voltage power unit is switched corresponding to a polarity of anion to be analyzed.
 10. A mass spectrometer using the high-voltageoutput by the high-voltage power unit according to claim 3 in an ionsource and/or an ion detector, wherein a polarity of a high-voltageoutput by the high-voltage power unit is switched corresponding to apolarity of an ion to be analyzed.
 11. A mass spectrometer using thehigh-voltage output by the high-voltage power unit according to claim 4in an ion source and/or an ion detector, wherein a polarity of ahigh-voltage output by the high-voltage power unit is switchedcorresponding to a polarity of an ion to be analyzed.